1. Field of the Invention
The present invention relates to a grinding machine with a truing apparatus for truing a grinding wheel made of cubic boron nitride.
2. Description of the Prior Art
In a grinding machine with a grinding wheel of such configuration that abrasive grain, made of a hard material, such as cubic boron nitride, is press-formed and stiffened on the outer peripheral surface of a base ring made of aluminum, there is provided a truing apparatus for truing such grinding wheel. In such grinding machine, it is very important to accurately control the in-feed amount of the truing apparatus against the grinding wheel. However, in the past, it was very difficult to stably control the in-feed amount within the accuracy of .+-. several microns. The main reasons for this difficulty are that the grinding wheel is thermally expanded by the change in the atmospheric temperature, coolant temperature and bearing temperature, resulting in a change in the relative positions between the truing wheel of the trying apparatus and the grinding wheel, and that the grinding wheel is slightly worn out by the grinding operations and this wear amount is not always constant.
For these reasons, the in-feed amount was not accurately controlled. When the in-feed amount is too large, not only the expensive grinding wheel is rapidly consumed, but also the metal removing ability of the grinding wheel is deteriorated, which may result in leaving a burn mark on the workpiece ground by such grinding wheel. On the other hand, when the in-feed amount is too small, the surface roughness of the grinding wheel is made worse, which affects the grinding accuracy and results in shortening of the truing interval.
The thermal expansion of the grinding wheel causes a change in the diameter of the grinding wheel which also affects the size of the ground workpiece, compensation therefor being very difficult.